Not answering mobile so unnerves one holding power (12)
I believe the answer is:
Here is my best explanation:
'not answering' is the definition.
(I've seen this before)
'mobile so unnerves one holding power' is the subsidiary indication.
'mobile' is an anagram indicator.
'one' becomes 'i' (roman numeral).
'holding' indicates putting letters inside.
'power' becomes 'p'.
'sounnervesi' anagrammed gives 'unresonsive'.
'unresonsive' placed around 'p' is 'unresponsive'.
(Other definitions for unresponsive that I've seen before include "disengaged", "Not reacting to stimuli" and "Not answering".)